Hi guys iv seen some wheels and I believe they are 8.5 ins all around what would be the best size for the rears if I want a wider tyre

Is up to you. I personally like original set up. I'm running on 225 45 17 and 255 40 17 and wheels front 17x8 and rears 17x8.5.
Then I think you will be fine with 255 at rear. As original bmw set up. I